Edge has a speech by S. Dehaene, Signatures of Consciousness. Here is a small part of it on the possible usefulness of consciousness. The whole speech is worth reading (here).
In several experiments, we have contrasted directly what you can do subliminally and what you can only do consciously. Our results suggest that one very important difference is the time duration over which you can hold on to information. If information is subliminal, it enters the system, creates a temporary activation, but quickly dies out. It does so in the space of about one second, a little bit more perhaps depending on the experiments, but it dies out very fast anyway. …. When you are conscious of information, however, you can hold on to it essentially for as long as you wish. It is now in your working memory, and is now meta-stable. The claim is that conscious information is reverberating in your brain, and this reverberating state includes a self-stabilizing loop that keeps the information stable over a long duration. Think of repeating a telephone number. If you stop attending to it, you lose it. But as long as you attend to it, you can keep it in mind.
Our model proposes that this is really one of the main functions of consciousness: to provide an internal space where you can perform thought experiments, as it were, in an isolated way, detached from the external world. You can select a stimulus that comes from the outside world, and then lock it into this internal global workspace. You may stop other inputs from getting in, and play with this mental representation in your mind for as long as you wish.
In fact, what we need is a sort of gate mechanism that decides which stimulus may enter, and which stimuli are to be blocked, because they are not relevant to current thoughts. There may be additional complications in this architecture, but you get the idea: a network that begins to regulate itself, only occasionally letting inputs enter.
It would be interesting to know how Dehaene separates the contents of consciousness from focus of attention. I do think that one of the reasons for consciousness has to do with being able to remember in episodic memory things that are in consciousness.